Nuclear receptors are a class of structurally related gene expression modulators that act as ligand-dependent transcription factors (R. M. Evans, Science 240, 889 (1988)). The steroid receptors, namely the androgen receptor, the estrogen receptor, the glucocorticoid receptor, the mineralocorticoid receptor, and the progesterone receptor represent a subclass of the nuclear receptor superfamily. Nuclear receptor ligands in this subclass exert their effects by binding to an intracellular steroid hormone receptor. After the receptor-ligand complex is translocated to the nucleus of the cell, the complex binds to recognition sites on DNA, which allows for the modulation of certain genes.
Certain substances have demonstrated the ability to exhibit their activity in a tissue selective manner. In other words, tissue selectivity allows a nuclear receptor ligand to function as an agonist in some tissues, while having no effect or even an antagonist effect in other tissues. The term “selective receptor modulator” (SRM) has been given to these molecules. A synthetic compound that binds to an intracellular receptor and mimics the effects of the native hormone is referred to as an agonist. A compound that inhibits the effect of the native hormone is called an antagonist. The term “modulators” refers to compounds that have a spectrum of activities ranging from full agonism to partial agonism to full antagonism. The molecular basis for this tissue selective activity is not completely understood. Without being limited to any particular explanation, particular ligands put nuclear receptors in different conformational states. These states dictate the ability of coactivators, corepressors, and other proteins to be recruited by the nuclear receptor (“NR”). The unique cofactor-NR ensembles are the gene transcription factors that are thought to modulate tissue selective effects.
Ligand-mediated effects through the action of nuclear receptors are not limited to the classical genotropic mechanism outlined above. It is thought that some, if not all, of the separation of anabolic and general homeostatic effects from the stimulation of sexual tissues can be explained by a particular ligand's ability to potentiate non-genotropic pathways. One example of liganded nuclear receptor induction of non-genotropic pathways is found in the work of S. C. Manolagas et al., Cell, 104, 719-730. The action of a sex steroid NR on osteoblasts and other cell types is shown to involve the Src/Shc/ERK signaling pathway. This activity is mediated through the ligand binding domain of the sex steroid nuclear receptor alone. The NR DNA-binding domain is not required to attenuate etoposide-induced apoptosis in HeLa cells. An NR lacking the DNA binding domain cannot function in the classical mode, acting as a transcription factor.
Nuclear receptor steroid ligands are known to play important roles in the health of both men and women. In regard to men's health, testosterone (T) and dihydrotestosterone (DHT), for example, are endogenous steroidal ligands for the androgen receptor that likely play a role in every tissue type found in the mammalian body. During the development of the fetus, androgens play a role in sexual differentiation and development of male sexual organs. Further sexual development is mediated by androgens during puberty. Androgens play diverse roles in the adult including stimulation and maintenance of male sexual accessory organs and maintenance of the musculoskeletal system. Cognitive function, sexuality, aggression, and mood are some of the behavioral aspects mediated by androgens. Androgens affect the skin, bone, and skeletal muscle, as well as blood lipids and blood cells.
The study of androgen action and male reproductive dysfunction continues to expand significantly. In fact, only recently has the definition of a disease state been associated with hormonal changes that occur in aging men. This syndrome, previously referred to as Andropause, has more recently been described as Androgen Deficiency in the Aging Male, or “ADAM” (A. Morales and J. L. Tenover, Urologic Clinics of North America (2002 Nov.) 29(4) 975). The onset of ADAM is unpredictable and its manifestations are subtle and variable. Clinical manifestations of ADAM include fatigue, depression, decreased libido, erectile dysfunction as well as changes in cognition and mood.
Published information indicates that androgen replacement therapy (ART) in men may have benefits in terms of improving body composition parameters (e.g. bone mineral density, increasing muscle mass, and strength) as well as improving libido and mood in some men. Therefore, andrologists and other specialists are increasingly using ART for the treatment of the symptoms of ADAM—though there is due caution given androgen's, like testosterone, potential side effects. Nonetheless, there is increasing scientific rational of and evidence for androgen deficiency and treatment in the aging male. Current testosterone-based ART therapies include injections, skin patches, gel-based formulations, and oral preparations. All of these therapies are somewhat efficacious in the treatment of ADAM, but, due to the dramatic fluctuations in plasma T-levels following treatment, success with these therapies has been variable.
Testosterone replacement products, such as AndroGel® (1% testosterone gel CIII, marketed by Solvay Pharmaceuticals) are emerging as a treatment of choice among physicians. Such products, however, fail to correctly mimic physiological testosterone levels and have potential side effects including exacerbation of pre-existing sleep apnoea, polycythemia, and/or gynaecomastia. Furthermore, the longer-term side effects on target organs such as the prostate or the cardiovascular system are yet to be fully elucidated. Importantly, the potential carcinogenic effects of testosterone on the prostate prevent many physicians from prescribing it to older men (i.e. age >60 years) who, ironically, stand to benefit most from treatment. Also, all of the existing treatment options have fundamental problems with their delivery mechanism. The need for a novel selective androgen receptor modulator (SARM) is obviated by the potential side effect profile manifested in conventional treatments. A SARM would ideally have all the beneficial effects of endogenous androgens, while sparing sexual accessory organs, specifically the prostate.
In regard to female health, progesterone, the endogenous ligand for the progesterone receptor (“PR”), plays an important role in female reproduction during the various stages of the ovarian cycle and during pregnancy. Among other things, progesterone prepares the endometrium for implantation, regulates the implantation process, and helps maintain pregnancy. The therapeutic use of synthetic versions of progesterone (progestins) stems from progesterone's ability to regulate endometrial proliferation. In fact, progestins are included as part of hormone replacement therapy (“HRT”) in women to reduce the incidence of endometriosis. Unfortunately, the effectiveness of therapy is tempered by undesired side-effect profiles. Chronic progestin therapy or continuous estrogen replacement regimens are often associated with increased bleeding. Excessive stimulatory effects on the endometrial vasculature may result in proliferation and fragility.
Compounds that modulate the effects of progesterone binding to PR are believed useful in the treatment and/or prophylaxis of endometriosis and uterine fibroid processes. Progesterone receptor antagonists such as mifepristone, also known as RU-486, and other PR modulators can inhibit endometrial proliferation at high estradiol concentrations in primates. Human clinical data with mifepristone supports the efficacy of a PR antagonist in endometriosis (D. R. Grow et al., J. Clin. Endocrin. Metab., 1996, 81). Despite enthusiasm for its use, RU-486 also acts as a potent ligand for the glucocorticoid receptor (“GR”). This cross-reactivity with the GR is associated with homeostatic imbalances.
Thus, modulators of nuclear steroid hormones that are highly specific for one receptor could offer greater benefit with less side effects in the treatment of both female and male related hormone responsive diseases.